"Say, 'Indeed, I fear, if I should disobey my Lord'"—by following you in what you call me to of idol worship—"the punishment of a tremendous day," meaning: Hell.
﴿قُلْ إِنِّي أَخَافُ إِنْ عَصَيْتُ رَبِّي﴾ بمتابعتكم عَلَى مَا تدعونني إِلَيْهِ من عبَادَة الْأَوْثَان ﴿عَذَابَ يَوْمٍ عَظِيم﴾ يَعْنِي: جَهَنَّم
